The present invention relates to an improved process for coating a substrate surface with titanium dioxide.
Sputtered coatings of various oxides (e.g. silica) and nitrides (e.g. silicon nitride) are used to form optical coatings showing interesting properties on a number of substrates. Known applications include low emissivity films on window glasses, cold mirrors on reflectors, enhanced mirrors for photocopiers and anti-reflective coatings on picture glass or TV screens. These coatings are usually made of stacks of several different layers with different refractive indices, preferably of low and high refractive index, to produce optical filters. For anti-reflective coatings it is preferred to combine two materials showing the highest and the lowest possible refractive indices. Such materials are titania and silica. Another advantage of these materials is their durability. For low emissivity films on window glasses it is preferred to combine a silver layer with a high refractive index material to dereflect the silver which improves light transmission.
Titanium dioxide coatings have a high refractive index and can thus be used to provide coatings of a high refractive index or to provide the high refractive index coatings in optical stacks. The existing process for producing titanium dioxide coatings comprises using titanium metal as the sputtering target and using oxygen as a component of the plasma gas. The titanium is thus converted to titanium dioxide during the sputtering process. Although satisfactory coatings of titanium dioxide can be produced, the rate of deposition is very slow and much slower than coating with zinc oxide and/or tin oxide. The stability of the sputtering process and the arc rate are both very dependent upon the conductivity of the target, particularly at high power levels.
As a substitute for titanium dioxide it has been suggested to use alternative materials such as niobium oxide. Whilst it is possible to coat a substrate with niobium oxide using a niobium metal target at slightly higher speeds than the equivalent process using titanium, niobium is very expensive.
JP-A-62-161945 describes a method of manufacturing a ceramic sputtering target in which a ceramic material consisting mainly of ZrO2, TiO2, SiO2, Ta2O3, Al2O3, Fe2O3 or a compound of these materials, is sprayed using a water plasma spray to produce a formed body which may be used as a sputtering target. The sputtering target is used for high frequency sputtering of non-conductive target material.
JP-A-1-118807 describes a method for manufacturing titanium oxide thin films in which a target comprising titanium, titanium monoxide (TiO), or titanium dioxide (TiO2) is used as a sputtering target and the target sputtered in a mixture of argon and oxygen using high frequency power density for example of 500 W.
There is, however, still a need for an improved process for coating titanium dioxide onto substrate materials. We have now surprisingly discovered that titanium dioxide can be sputtered from a target comprising sub-stoichiometric titanium dioxide of low resistivity of less than 1 ohm.cm, preferably less than 0.1 ohm.cm to provide coatings on a substrate either of sub-stoichiometric titanium dioxide, or titanium dioxide, depending upon the sputtering conditions.
Accordingly, the present invention provides a process for coating a substrate surface with titanium dioxide, which process comprises the D.C. plasma sputtering and/or mid-frequency sputtering from a sputtering target which comprises sub-stoichiometric titanium dioxide, TiOx, where x is below 2, and which has a resistivity of below 1 ohm.cm.
Sub-stoichiometric titanium dioxide, TiOx, where x is below 2 and generally is in the range of from 1.55 to 1.95 is known in the art. It is a form of titanium dioxide which is conductive. Preferably the sub-stoichiometric titanium dioxide used in the present invention has a resistivity of below 0.1 ohm.cm.
The sputtering target which is used in the process of the present invention comprises sub-stoichiometric titanium dioxide, TiO coated onto a target base, such as a backing tube or plate, for example a target base of an electrically conductive material, for example stainless steel or titanium metal, aluminium or copper. Because the sub-stoichiometric titanium dioxide is conductive, D.C. plasma sputtering and/or mid-frequency sputtering, such as the Twin-Mag System may be used. D.C. plasma sputtering is, however, preferred. The target may be of any type known in the art, for example a rotatable target or a flat magnetron target.
The sputtering target used in the process of the present invention may be prepared by plasma spraying titanium dioxide, optionally together with niobium oxide, onto a target base in an atmosphere which is oxygen deficient and which does not contain oxygen-containing compounds, the target base being coated with TiOx, where x is below 2, which is solidified under conditions which prevent the sub-stoichiometric titanium oxide from combining with oxygen. During the plasma spraying process, the action of the plasma on the titanium dioxide causes the titanium dioxide to lose some oxygen atoms from its lattice preferably from the surface of the particles. The titanium dioxide is converted into the sub-stoichiometric form, i.e. non-stoichiometric oxygen deficient titania The sputtering from the sub-stoichiometric titanium dioxide, TiOx target is preferably carried out using as the plasma gas argon, a mixture of argon and oxygen, a mixture of nitrogen and argon, or a mixture of nitrogen and oxygen. If the plasma gas does not contain oxygen, e.g. if pure argon is used, then the coating will comprise sub-stoichiometric titanium dioxide. The coating obtained is not completely transparent and possesses some conductivity. If, however, the plasma gas contains oxygen then the sub-stoichiometric form of titanium dioxide is converted during the sputtering process into the transparent form which is stoichiometric or substantially stoichiometric and has a high refractive index. The degree of transparency will depend upon the amount of oxygen contained in the plasma gas. A preferred gas mixture to form transparent titanium dioxide as the coating comprises 70-90% by volume argon and 30-10% by volume of oxygen.
The substrate which is coated according to this process may comprise, for example, optical glass, the screen of a cathode ray tube, such as a TV screen, cold mirrors, low-emissivity glasses, architectural glasses, anti-reflective panels, or flexible films such as oxygen barrier films.
The use of the target of the present invention avoids any avalanche effect of titania towards the metallic state or poisoned oxygen state. There is no need for a sophisticated gas control system or plasma monitoring equipment. The target can be easily operated and controlled to produce stoichiometric or non-stoichiometric films of any level of sub-stoichiometry. By operating the target in a slightly oxygen deficient sputtering process, a high refractive index film can be obtained which is important for optical coatings. Depending on the sputtering parameters, amorphous or crystalline films can be formed with a rutile or anatase structure. Rutile films have improved optical, mechanical and electrical properties. When the sub-stoichiometric titanium dioxide is sputtered under conditions such that the coating is formed comprises stoichiometric titanium dioxide, the coating is essentially transparent and colourless.
Although films coated with stoichiometric titanium dioxide, for example for use in the packaging industry, have good oxygen barrier properties, they are statically loaded which is particularly disadvantageous especially for the packaging of powdered products, such as coffee.
When the coating formed from the sub-stoichiometric titanium dioxide sputtering targets is also sub-stoichiometric titanium dioxide, it has a reduced transparency and slightly blue colour but is conductive and thus possesses anti-static properties having an electrical resistivity of above 3xc3x97105 ohm.cm. Accordingly, films coated with nonstoichiometric titanium dioxide not only have good oxygen barrier properties, but also possess particularly good anti-static properties which makes them particularly suitable for packaging applications, particularly in the food industry. A further advantage of films coated with sub-stoichiometric titanium dioxide is that the coatings possess good flexibility which even after excessive bending and creasing retain their oxygen barrier properties. Although the films have a slight blue colour this does not detract from the potential use of the product in the food industry, the blue colour providing a xe2x80x9cfreshxe2x80x9d look to the product. As barrier films for water vapour and oxygen, there is an improvement when compared with the prior art titanium dioxide coated films of five fold for water vapour and three fold for oxygen.
The main advantage of the present invention is that from the sub-stoichiometric titanium dioxide targets used in the present invention the rate of sputtering is increased by a factor of about ten as compared to sputtering from a titanium metal target, thus making the process industrially attractive. Furthermore, the sputtering process is very stable with little or no arcing occurring.